(CNN) -- Four NATO troops were killed Sunday in an attack believed to involve Afghan police in southern Afghanistan, officials said, in what appears to be the latest incident of local forces turning their guns on coalition partners.

The troops died "following an insider attack involving suspected Afghan police," said Jamie Graybeal, a spokesman with NATO's International Security Assistance Force. No more details were provided.

The attacks followed news that insurgents disguised in U.S. Army uniforms carried out a coordinated assault Friday at a joint American-British base in the same region. That assault raised concerns that the attackers had inside knowledge.

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It is too early to conclude whether the attackers had "inside knowledge," Graybeal said. The i